Fixed-Price vs Cost-Plus Building Contracts: How Each Changes Your Construction Loan

In short: A fixed-price building contract lets a lender size your construction loan against a known cost, follow the contract's progress payment schedule and ask for little contingency. A cost-plus contract leaves the final cost open, so lenders typically load the estimate with a contingency, test serviceability on a bigger limit, inspect more often and may lend at a lower LVR. Provisional sums and prime cost items sit in between: the more of them, the more a fixed price behaves like cost-plus.

Key takeaways

Under a fixed-price contract the builder carries most cost movement during the build; under cost-plus, you do.
Lenders usually size a fixed-price loan on the land debt plus the contract price, and a cost-plus loan on the estimate plus a contingency loading.
Provisional sums and prime cost items are allowances, not prices - large ones push a fixed-price contract towards cost-plus risk.
Cost-plus draws are paid against invoices and need more frequent inspections, which adds fees and paperwork.
Send the draft contract to the lender before you sign; a contract changed after the credit decision can restart the assessment.

What is the difference between a fixed-price and a cost-plus contract?

A fixed-price (lump-sum) contract sets one price for a defined scope of work. The price moves only through variations you agree to, adjustments to provisional sums and prime cost items once the real cost is known, and any rise-and-fall clause the contract contains and your state allows. A cost-plus contract has no fixed total: you pay the builder's actual cost of labour, materials and subcontractors, plus a margin that is either a percentage of cost or a fixed fee. The final figure is known only when the last invoice arrives.

Several states restrict cost-plus contracts for domestic building work or require extra disclosure, so check your state's building regulator before you choose one.

Feature	Fixed-price	Cost-plus
Total price	Set at signing, subject to variations and allowances	Unknown until the end
Who carries cost increases during the build	Mainly the builder	You
Builder's incentive	Control cost to protect the margin	A percentage margin grows as cost grows
Paperwork	Progress claims against stages	Invoices, timesheets and supplier statements
Typical use	Project homes and standard designs	Custom homes, difficult sites, renovations, some commercial work
Lender view	Easier to fund	Harder to fund, with more conditions

Why do lenders prefer fixed-price contracts?

Because they can see the end. A lender sizing a construction loan wants three numbers before the first draw: what the land is worth, what the build will cost, and what the finished property will be worth. A fixed-price contract supplies the second number in writing, signed by a licensed builder who is obliged to deliver for it.

That certainty flows through the file. The valuer checks the contract price against the as-if-complete value. The credit team sizes the limit at the land debt plus the contract price, sometimes with a small allowance for provisional sums. Draws follow the contract's payment schedule, so the lender knows in advance what each stage will release.

Cost movement matters more than it did a few years ago. The ABS Producer Price Indexes for the June quarter 2026 show house construction output prices up 5.9% over twelve months. Under a fixed-price contract, most movement during your build is the builder's problem, within the contract's terms. Under cost-plus, every increase lands on your loan.

A fixed price is only as good as the builder who signs it. Check the company on ASIC's registers, including published insolvency notices, and confirm its ABN and GST status on ABN Lookup before you sign.

How do lenders treat provisional sums and prime cost items?

A prime cost (PC) item is an allowance for something you have not yet chosen: tapware, appliances, tiles, light fittings. A provisional sum (PS) is an allowance for work whose cost cannot yet be known, such as rock excavation, retaining walls or service connections. Both are estimates inside a fixed-price contract. If the real cost is higher, you pay the difference, usually plus the builder's margin on the excess.

Lenders read the allowances as a measure of how fixed the price really is. A $650,000 contract with $15,000 of allowances is close to certain. The same contract with $120,000 of allowances, much of it site works, behaves more like cost-plus, and the credit team may add a contingency or ask you to show cash to cover overruns.

Allowance	Example	Risk to the budget	Typical lender response (indicative)
PC items, selections made	Appliances chosen and priced	Low	Accepts as priced
PC items, selections open	Tiles at an allowance per square metre	Medium: upgrades are easy to make	Asks for a selections schedule; may add a buffer
Provisional sums, site works	Rock excavation, retaining	High: unknown until digging starts	Wants a soil report or a cash buffer
Provisional sums, services	Sewer, power and water connections	Medium	Wants authority quotes where available

The most effective way to tighten a contract before you take it to a lender is to make your selections and complete site investigations first, so allowances become fixed prices.

Fixed-price vs cost-plus: a worked comparison

For illustration only - example figures, not any lender's policy or an offer. The same house on the same block. The land is valued at $500,000 with a $200,000 loan on it, and a valuer puts the as-if-complete value at $1,250,000. Builder A offers a fixed price of $650,000. Builder B offers cost-plus, estimating $580,000 of cost plus a 10% margin, or $638,000.

Line	Fixed-price (Builder A)	Cost-plus (Builder B)
Price or estimate	$650,000 fixed	$638,000 estimate
Lender's contingency loading (example)	None	15%, or $95,700
Build cost the lender plans for	$650,000	$733,700
Plus the existing land loan	$200,000	$200,000
Funding to be covered	$850,000	$933,700
Maximum LVR on $1,250,000 (example)	80%, or $1,000,000	70%, or $875,000
Loan	$850,000 (68% LVR)	$875,000 (70% LVR)
Your cash before the draws start	$0	$58,700

Builder B's estimate is $12,000 lower, yet the cost-plus loan needs $58,700 of your cash and a larger limit. The lender in this example loads the estimate by 15%, plans for $733,700, and caps the loan at 70% of the end value instead of 80%, so $933,700 of funding meets an $875,000 maximum loan.

Serviceability moves too. Banks assess repayments at least 3 percentage points above the loan rate, the buffer APRA left unchanged on 28 May 2026. At an example rate of 6.5%, assessed at 9.5% over 30 years, the $850,000 limit tests at about $7,147 a month and the $875,000 limit at about $7,357, so the cost-plus file needs about $210 a month more surplus income. Our guide to why borrowing power varies between lenders explains how each lender's calculator treats that.

How do progress draws differ under each contract?

Under a fixed-price contract, the builder claims a set percentage of the contract price at each stage: a deposit, then base, frame, lock-up, fixing and practical completion. The valuer confirms the stage is reached and the lender pays the claim. Many states regulate deposits and stage payments on domestic contracts, so schedules are fairly standard and lenders know them well.

Under cost-plus, the builder invoices costs as they are incurred, often monthly, plus the margin. The invoices do not line up with stages, so the lender needs someone - a valuer for a house, a quantity surveyor for larger or commercial jobs - to confirm each time that the work in place matches the money spent and that the undrawn funds still cover the cost to complete.

Point	Fixed-price	Cost-plus
Draw trigger	Stage reached	Invoice period ends
Evidence	Builder's progress claim and valuer's inspection	Invoices, supplier statements, timesheets, QS or valuer report
Number of draws	Usually around six	Often many more
Inspection fees	Per stage	Per draw, so often higher in total
Cost-to-complete test	Largely set by the contract	Run at every draw

See how the timing of each draw changes your interest bill with the construction progress draw calculator, and read how construction loan progress payments work on a standard contract.

Why is a cost-plus construction loan harder to fund?

The limit is an estimate. The lender is asked to commit to a number the contract does not fix, so it protects itself with contingency loadings and lower LVRs.
The incentive runs the wrong way. A percentage margin means the builder earns more when costs rise. Lenders prefer a fixed fee or a capped cost.
Overruns arrive late. Cost growth often shows at fixing and finishing, when most of the loan is drawn and the options for topping up are narrow.
More administration. Every draw needs invoice checking and a cost-to-complete review, which some lenders will not resource for a residential loan.

Some banks decline cost-plus contracts for residential construction, or accept them only with a maximum-price cap. Non-bank and private lenders are more likely to consider them, generally at lower LVRs and higher cost. On investment and commercial builds, a quantity surveyor's cost plan and monthly monitoring are standard; our guide to construction and development finance covers how larger projects are monitored.

How do you negotiate the contract with the loan in mind?

Ask for a fixed price for the scope you have decided, and put the undecided parts into clearly priced allowances.
Make your selections before signing, so PC items carry real prices.
Commission soil and survey work early, so site-work provisional sums shrink.
If you choose cost-plus, negotiate a fixed fee instead of a percentage margin, open-book invoicing and a maximum-price cap.
Read any rise-and-fall clause and ask exactly how it is calculated.
Match the contract's payment stages to the lender's draw stages.
Send the draft contract to the lender or broker before you sign.

Common mistakes: choosing the lower estimate without pricing the lender's contingency; signing with large open allowances and assuming the loan will stretch; and agreeing variations mid-build without checking that the undrawn balance still covers the new cost to complete.

What should you do next?

If your builder offers both, get both contract types priced and set them side by side as a lender would: price or estimate, allowances, contingency, loan limit, cash needed and assessed repayment. Test the repayment on the full limit with our guide to stress-testing your budget at +3%.

When the contract is settled, post one construction loan request on ADS and 200+ verified lenders and brokers can ask about it. This is general information, not legal advice; have a building lawyer review the contract before you sign.

Frequently asked questions

Can I get a construction loan with a cost-plus contract?

Some lenders fund cost-plus contracts and some do not, particularly for owner-occupied homes. Those that do usually load the estimate with a contingency, lend at a lower LVR, want a detailed cost estimate and inspect more often. A maximum-price cap, a fixed builder's fee and open-book invoicing all make a cost-plus file easier to fund.

What is a provisional sum in a building contract?

An allowance for work whose cost is not yet known, such as rock excavation or service connections. If the actual cost is higher, you usually pay the difference plus the builder's margin. Lenders treat large provisional sums as uncertainty and may ask for a contingency to cover them.

What is the difference between a prime cost item and a provisional sum?

A prime cost item is an allowance for something you have not yet selected, like tapware or appliances. A provisional sum is an allowance for work whose cost cannot yet be measured, like excavation. Both can rise. Making selections and completing site investigations before signing turns both into fixed prices.

Does a fixed-price contract mean my build cost cannot change?

No. The price can still move through variations you request, adjustments to provisional sums and prime cost items, and any rise-and-fall clause. Each change can affect your loan, so check that the undrawn balance still covers the cost to complete before you agree to a variation.

Should I show the lender my building contract before I sign it?

Yes. The valuation and loan size depend on the contract, its stages and its allowances, so showing the draft first lets you fix problems before they are locked in. A contract signed or changed after the credit decision may need a fresh valuation and assessment.

Sources

Producer Price Indexes, Australia, June quarter 2026 (house construction output prices) — Australian Bureau of Statistics
Search ASIC registers, including published insolvency notices — Australian Securities and Investments Commission
ABN Lookup: ABN status and GST registration — Australian Business Register (business.gov.au)
APRA maintains macroprudential settings: 3 percentage point serviceability buffer (28 May 2026) — Australian Prudential Regulation Authority

Information current as at 24 October 2026.

General advice warning: This article contains general information only. It does not take into account your objectives, financial situation or needs, and it is not personal credit or financial advice. Consider whether it is appropriate for you and seek advice from a licensed credit representative before acting.

Any interest rate shown is an example only and is not an offer of credit. Where a rate is quoted, the applicable comparison rate is available from the relevant lender and should be considered alongside it.